Gain-controlling system for seismographs



. May 13, 1947 E. J. sHlMl-:K Er AL 2,420,571

GAIN-CONTROLLING SYSTEM FOR SEISMOGRAPHS Filed June 7. 1944 5 Sheets-Sheet 1 May13,1941. E.J.SH|MEK Em '2,420,511

GAIN-CONTROLLING SYSTEM FOR SEISMOGRAPHS Filed June 7, 1944 5 Sheets-Sheet 2 JNVENToRs. raw/N J. SH/MEK 60er/le l. iufm-Marre or-ney.

May 13, 1947.

E. J. SHIMEK ET AL GAIN-CONTROLLING SYSTEM FOR SEISMOGRAPHS Filed June '7, 1944 5 Sheets-Sheet 4 nin-W IN VEN TORS .5H/Mer 5y. g l

/for-nex May 13, 1947.

E. J. SVHIMEK ET AL GAIN-CONTROLLING SYSTEM FOR SEISMOGRAPHS Filed June 7. 1944 5 Sheets-Sheet 5 INVENTORS. fam/v l ff/15K 6am/e M Gro-Navarre Patented May 13, 1947 GAIN-CONTROLLING SYSTEM FOR sEIsMoGRAPns Edwin J. Shimek, Dallas, Tex., and Goethe M. Groenendyke, Norman, Okla., assignors, by mesne assignments, to Socony-Vacuum Oil yCompany, Incorporated, New York, N. Y., a

corporation of New York Application June 7, 1944, Serial No. 539,146

1o claims. (ci. 177-352) l This invention relates to electric'seisrnographs, more particularly to a method and apparatus for controlling the gain in amplification throughout the period of time during which seismic waves' created in the earths surface are being detected and amplified, and has for an object the provision of a system in which there are provided independent adjustments of the amplification characteristics.

A number of methods for controlling the gai in amplification, Vas derived from thermionic amplifiers, is well known in the art. For example, in Shimek Patent No. 2,317,334, now Reissue Patent No. 22,535, a system is disclosed in which the initial amplification is maintained at a maximum value until signals corresponding with the direct traveling seismic waves have been recorded. The gain is then automatically reduced to a predetermined value in order that reflections of substantial amplitude from relatively shallow interfaces may be recorded on a limited Width of recorder strip. The gain in amplification is then caused to increase with time, the purpose being to compensate for the loss in amplitude of the reflections from the deeper strata. While this system represents -a substantial advance in the art, the present invention further extends the usefulness of such methods and apparatus for controlling the gain in amplification,

More specifically, there are provided, with a minimum of equipment, a method and means for changing the initial level of amplification without changing the time either for the reduction of amplification, or the time at which the amplification again becomes a maximum. In terms of aiseismic survey, this means that the degree of suppression or contraction of the amplifica-I tion maybe selected in any desired manner, and, in particular, to conform with the amplitude of the direct traveling waves and their refiections. The amplitudes of the reflected Waves will of course vary according to the character of the subsurface strata giving rise to them. Where maximum gain is required for them it isfpossible to reduce the initial level of amplification Without affecting the time at which amplification again reaches a maximum. Further, in accordance with the invention, the time at which the ampllcation again becomes a maximum may be changed without affecting theinitial level of amplification or the degree of contraction of arnplificatioirwhich may be desired. In terms of a seismic survey, it is emphasized that the expansion of the amplification should conform with the attenuation of the reflected seismic waves. In-

asmuch as the attenuation will differ invarious o" localities, the independent adjustments provided are highly advantageous. Y Further, in accordance with the invention,

there are provided, with a minimum of; equip.` ment, a method and means for varying the time f rate of change in the gain or expansion ofthe amplification without changing the overall time from minimum to maximum amplification. This feature is particularly useful where the'inte'rmef f diate reflections in different localities have maf-*fh terially differing amplitudes but where the reflec` tions from the deeper strata require maximum gain in a time interval less than that whichV`Y would bepro'vided with adjustments which would I ofexpansion'- take care of the intermediate period o-f amplification. v

For a more detailed description of the inven tion, and for further objects and advantages thereof, reference should be had to the following detailed description, taken in conjunction' with the accompanying drawings in which: t

system;

Fig. 2 is a graphk iuustrating three different initial values of the gain-controlling voltage Va, but in which the time of maximum is the same in each case;

Fig. 3 is a graph. illustrating three dierent' rates of rise of the amplification-controlling bias Y Vs with the initial contraction-of amplification,

or the initial value of Vs, the same; r

Fig. 4 is a simplified wiring diagram illustrat-l ing certain additional features of the invention; Fig. 5 is a graph illustrating one way in which the gain-controlling voltage V8 maybe caused to vary; l

Fig. 6 is a graph illustrating the overall ampli- 4 fication characteristic with VB varying, as shown in Fig, 5;

Fig. 7 is a simplified wiring diagram illustrating certain additional features of the invention;

Fig. 8 is a graph illustrating the manner in which Vs may vary at different rates;

Fig. 9 is a graph illustrating the overall amplification characteristics with Vs varying, as shown in Fig.8;

Fig. 10 is a Wiring diagram of the system as a whole and in which there are included the features of Figs. 1, 4 and 7;

Figs. 11 and 12 aregraphs illustrating changes in operational characteristics due to movement of l OFFICE- f amplification the-.instantof explosion of the charge. This imy pulse corresponds with the instant Aof creation of the seismic waves, and it is recorded on the seisinograph as the time break.

, In order to show the essentials of the invention in `their simplestform, only selected parts of the system have been illustrated in Figs. l, 4 and '1. These will rst be described in detail and then tlie` system as a whole, as shownln Fig. 10, will lie-explained.

o' Referring now to Fig. ,1, the invention is shown asapplled to an Aamplifier I and a geophone Il, which'is coupled by transformer I2 to the pentode I3." ,It is to be understood that additional geophones'corresponding with geophone II are distributed along a selected area of the earths surface, anditogether are known as the spread.. Each'of the. spreadgeophones may be provided with an amplifier lll and each amplifier may incldeonel or more electric yalves, only one of which, the pentode I3, is shownin Fig. l. Preferablyvtwo pentode tubes are provided, connected in cascade, each of which' is provided with a suppressorgrid I4. The amplifier Ill'may be of conventional design. As indicated, a coupling capacitor I couples the pentode I3 to the next stageof `the amplifier, and cathode bias for the control grid of tube I3 is provided as indicated at I6.' `Seismic signals received or detected-by the' gecphone II are applied to the input circuit of thepentode I3 by means of the transformer I2. Amplication of these signals is controlled by suital'llyA varying a negative bias VB applied to the suppressor grids I4 through a biasing circuit, whichfincludes th'e conductor I1 and the ground connection G. The output from the amplifier IU i`s=-a`ppliedto a recorder I8 which may be of any suitable type, such, for example, as a multi-elenient recording galvanometer. Each element has afrnirror; movable in response to theiseismic signais, to produce on a seismogram a trace of oscllograpliic record of limited width of the signals detected bythe geophone lI.

At ,theiinstant of creation of the seismic waves al voltage impulse is applied to the input terminals 20and2l of a transformer 22. This impulse is of `such-polarity as to reduce a negative bias, applied byja'battery 23, to thecontrol grid `o1? a gas tube 24;" The gastubeZl may be of a type known on the market'by the' trade name of Thyratronff andfhaving the `characteristic of continued flow of anode current once the control grid loses control. `Hence, upon reduction of the negative grid biasby the applied impulse, the Thyratron lires and current flows from a battery 26th'rough the gas-tube and through a resistor 21. The resultant IR drop, or potential difference across the resistor 21, is applied'by conductors 32a and 32h to: a' network comprising capacitors 28 and 29 and resistors .'30k and 3l.

iIt isfto'be observed th'e upper end of resistor is connected byconductor 32 to ground G and by` conductor 32a to a positive tap of battery 2i: Thus, before thetube 24 res, a negative biasVs is applied to the suppressor grid I4. Its magnitude depends upon the value of the volttactor 33. As the contactor 33 is moved toward' conductor 32 the initial negative value of Vs is decreased. The capacitors 28 and 29 are initially charged'with the upper plates positive and the lowerplates negative, as viewed in Fig. l. Upon flow of current through the resistor 21, however, the resulting potential difference tends to reverse the polarity of capacitors 28 and 29. They lose their charge at a rate dependent upon the setting of the contactor 34 on' the resistor 30. As the contactor 34 is moved upwardly, a greater pro-- portion of the resistor 3D will be included in circuit with the capacitors 28 and 29. This has the eiect of increasing the time required for them to lose their initial charge and to reverse in polarity. As the voltage across the capacitors 28 and 2B decreases, the voltage across the resistors 30 and 3| also decreases at a corresponding rate from its initial negative value.

The foregoing operation is graphicallyfillustrated in Figs. 2 and 3. The graphs 33a, 33band 33e of Fig. 2, plotted with time as abscissae and'v firing ofthe Thyratron 24 this initial negativeY bias will decrease, as indicated by the curve 33a, reaching its zero value at time T2. Initially, this high negative bias reduces the gain of the amplifier to a predetermined or preselected low value, such, for example, as would be desired for the reception of shallow reflections or direct traveling seismic waves. During the time interval from zero to T2 the negative suppressor grid bias Vs decreases to zero and the amplification is correspondingly increased. The amplification reaches a maximum as the suppressor grid bias approaches zero. The amplification is not increased as the suppressor grid bias is made positive with respect to the cathode. The relation between amplification and suppressor grid bias is well understood in the art, and reference may be had to Minton et al. Patent No. 2,301,739, for

.illustrative graphs with respect thereto.

An important feature of the present invention is that adjustment of the contactor 33 is effective in changing the initial negative bias without changing the time, such as the time T2. at which the amplification reaches a maximum. Thus, if the contactor 33 is moved upwardly by a predetermined amount, as viewed in Fig. 1, the initial negative bias will be reduced to a value, as indicated at 31. Upon firing of the Thyratron 24, however, the negative bias will decrease, as indicated by the curve 33h, with the time of maximum amplification attained at T2, as before. Similarly, if the contactor 33 is moved to include still less resistance, the initial value of the negative bias will be as indicated at 38 and again maximumamplication will be attained during the same nite time interval, as indicated at T2.l It may be observed that the various curves 33a to 33e intersect and cross each other at thepoint T2, but this fact is immaterial inasmuch as a positive bias on the suppressor grid does not i chaman:

trast` with prior suggestions where a capacitor. is

discharged througha resistor.

The reason .the adjustmentofthe contactor33;`

with respect to the resistor 3|; does not affectthe.

time .i at which the amplifcationwill be a maxi.

mum is that. the` total resistance of',` resistorsl 30..

and 3|*remains unchangedzinsofar as the appliedV potentialv diierences are concerned. In other Worcls,` the parameters ofthe time-delay circuit, including the` capacitors 28` and 29, are not changedby movement of the contacter 33.

Similarly, adjustment of the contactor 34, with respect tothefresistor` 30'; doesnot changethe. parameters of the circuit whichincludes theconductorlland. ground connection G. However,`

asshovvn` inrlig.A 3, the adjustment of the con-v tactor 34W does change the. resistance in the chargingv circuit of; the 'capacitors 28 andy 29;;

Forrcomparative purposes the curve 33h of Fig. 2i

is` reproduced in Fig. 3` as curve 34h. It repre-4 sentsthe conditions with thecontactors in the positions shown in Fig. 1. Ifthe contacter 3,4.be

moved downwardly to decrease the amounttof'Y resistance in series with the capacitors 281 and 29;: the maximum negative bias .indicated i at 31* remains unchanged. However, upon the firing of theThyratron 24 the capacitors 28 and 2 9 Iwill discharge more rapidly, and after reversal` oftheirpolarity, will be more rapidly charged, asfindicatedby theicurve-34a- If the contactor 34;.`

be movedupwardly to include more resistance in the time-controlling circuit, the rise in voltage is; at a lower rate, as shown by the curve 34e. Thus, adjustment of the contacter 34 changes theY rate atwhich the suppressor grid bias Vs decreases` from a predetermined negative value to zero, and thence to a positive value. The result, of course,

is a` change inthe time intervalrequired for Vs.

to be reduced to zero. The times at which maximum.. amplicaticn occurs for the three curves 34a, 34h and 34care indicated at.T|, T2 and T3..

' It will now beseenthat there-is provided in dependent control of the extent of the initial contraction of` the amplificationand the time `A rate at which the amplication increases or expandsr-V It is, therefore, possible; to preset thel amplifier strictly in accordancewith the amplitudecharacteristics ofthe seismic waves detected by the spread geophones, whetherthey be direct travelingor reected waves. Either the extent ofthe initial contraction, or the time, from the instant of contraction to maximum amplification maybe adjusted independently of each other, or both, may be adjusted between relatively wide limits to meetv widely varying field requirements. Irithisrmanner, improved records are attained in less time, records which. may be more readily interpreted and which, because of relatively exact compensation for changes of--amplitude, may be readmore accurately and withrelatively greater easethan has heretofore been possible.

Referring now to Fig. 4, provision is also made for initial operation, with the amplifier setifor maximum gain. This isvdesirable in most seismic surveying systems in order that the first breaks .65.

which appear atl the spread geophones may be recorded with maximum accuracy. The first breaks correspond with the instant of arrival at the respectiveV geophones of the direct traveling seismic waves. These Wavestravel directly'from the shotpoint through the upper and weathered layer ofthe earthsV surface.

travel time from the shotpoint to the respective geophones, calculations may be made to ascertain the velocity of the waves through the weath-.

By ascertaining the a ered.;` laver. Inasmuch as the.` instant en -arrival. of these waves .is.:the= principal thi-ng, offinterestri the recordation, with imaximum gaimby;thaarnf. Ly plier, produces 1a.sharper break.; than. would'sbe.r` the case if y the amplilcationfwere.; substantially i reduced. l That is.to say. theapnlication tothe.

galvanometer loira.. signal; or; substantialr'mag-nitude produces. anx abrupt: movement ofthe; gall vanometer coil; andlthe galvanometer mirror-in L turn causes the light lbeam on theglmor, seismogram to? move y fromfits resting positiciir abruptly-- in one direction-.or thetothers VEven thQug'hLthefj. lightf beam- `is nfiovedsv beyond 1 the; confines ;offlthey relatively `narrowreeord er;strips lisci/cr.thelessfthel 15".

itsk resting -pos1tion,p1fovidesfclearand.aeeurate.-

indication of the instantyof; arrival? offthe i directtraveling waves..

Asshownfingligsi; it will-bev observedthat."A the lower end offfthe resistorfat is connected..by i vconductor.4Ilitoga normally;openrcontact;4i ofa high speed, relay,` diagrammatically. il.lfllstlfatedfl at 42.` Untir tne..re1ay. liz-15. operated 'td/come@ plete a. circuit through the normallyjopen con-' through the lresistorv 4l!to the conductor 32'.` The .Y

resistors 3|: and 44sareiidentical.' Hence,jso far-j as the circuit is concerned, intermsoffthe.oper-5 ating characteristics. described -in connection With Figs. 1-3,` the; operation is identicalwithfeitherI V resistor 4 4VA in series `withthe resistor 3| or the the resistor 30.

It will be understood*that/only` av relatively# short time interval isr required .for4 the. direct traveling Wave to reach thefspread.geophones.v

After expiration of thisttime interval, the relay 42 is energized to open thecircuit-ai; thecon tact l43-` and to Vclose the circuit through'4 the con tact 4|. Thel resistor23|lis. therehy substitutedv in the circuit in place ofthe' resistor 44. Hence, the voltage which appearedacross theresistorfMv atonce appears across the.resistor-3|, and isy applied through the conductor` vand the ground connection -G to the suppressor-gridssubstantiallyv instantaneously to contract or-to reducel the amplicationor gainin the amplifier: The` resistor` 445 is,l in i effect, adummy; resistor rwhich permits the removalU of resistor 3 Without aff fecting the circuitv constants ofthe.` controlling@ Y Asabiove explained, at the, instantofz detonation Otthebharge, the Thyratron- 2 4f .res

circuits.

and a voltage,` orpotential difference, immediate-' ly appearsacross `theresistor 2 1. A. part ofthatvoltage; isA applied toa circuit, which` maybe traced from ai part of.; the resistor 2T by. the; switch 46; adjustablecontactor4'l1 of a resistor.4

48; a capacitor 49, and by conductor'ftoythe other,` side of resistor 21. The resiston 4 8v con-.;

trols the charging time ofthe-capacitor, which; it will be observed, is connectedin circuit with;

a bias battery 5|` and a resistor .sz-in trie-grid circuit of a Thyratron 53., Thebias battery 5| normally negatively biases the Thyratron 53,;t0` prevent iiow of anode current. However, as theV .voltage rises across the condenser 49 it is ofi1 7 such polarity as to reduce the negative bias from battery 6|. At a predetermined time interval the bias is reduced to a point where the Thyratron 53 fires. positive end of battery 26 by way oi' conductor 54, the operating winding of the relay 42, con- Current then flows from the ductor `55, through the'Thyratron 53, and by v conductor |Ijto' the negative side of battery 26. The high speed relay 42 is, in this manner,

justed tosuit the requirements ofthe particular,

seismicsurvey'being conducted. Any particular setting will depend upon the spacing of the spread geophones from the shothole, and upon the velocity characteristics of the weathered layer in the'region being explored. Y

Graphically, the foregoing operations arevillustrated'b'y the graph of Fig. 5 of the suppressor grid bias, and the graph of Fig. 6 of the corresponding changes in the over-al1 amplification of the amplifier.v At the instant of detonation, the suppressor grid voltage Vs is zero, as indicated at To, which corresponds with Amaximum ampliiication.lv by the amplifier. After a time interval, adequateforthe direct traveling waves to reach the spread geophons, the Thyratron 53 fires, as at the time' T4, `and the high speed relay 42 operates' to 'connect the resistor 3| in the circuit. The gain-scontrollingbias is then immediately applied to theampliiien'as indicated at 51. The effect fis=4 immediately to reduce or contract the amplification to arpredetermined value, as indicated at T58, Fig. '6L Thereafter, the operation is as described above, and the ampliilcation is graduallyincreased, as indicated by the full line curve 59, `which corresponds with aldecreasing negative bias, as indicated by the full line curve 60 of Fig. 5. By adjusting the position of the contactoru it win be observedthat the instant at which the contraction occurs may be changed.

If it is later than as shown in Fig. 6, thenthe' degree o'f contraction will be less. In other words, at'the time T4 the contraction indicated at 58 fis less than at the time To. If greater contraction is desired at the time T4 it will be recalledl that the contactor 33 may be moved in the correct 'direction' (downwardly in Figs. 1 and 4), negatively 'to increase the gain-controlling voltage Vs. It will also be remembered this adjustment does not affect the other operating characteristics, particularly the time at which maximum amplification is attained. Neither does the value of the time assigned to T4 alter the magnitude of Vs which would have existed at timey T4 had the first break feature not been operative.

In'certainareas in which geophysical surveys `are made, the geological structure may comprise relatively shallow reflecting interfaces followed yao shown in Fig. '1, as applied to the simplified circuit forming a part of Fig. 1. 'I'hese provisions comprise a high speed relay, diagrammatically shown at 63,y with a normally closed contact 64 inserted in series with the capacitor. VvAs long as the contact 64 remains'closed, the operation is identical, as described above in connection with Figs. 1 and 4. Specifically, referring to Fig. 8, the gaincontrolling voltage Vs decreases from a maximum negative value 36 in accordance with the characteristic curve 65. At a time T6 it is desired to have lthe amplification at a maximum. Therefore, at a time T5V the biasis rapidly decreased along the broken line curve Eli-"reaching zero at T6, this value corresponding with maximum amplification. In terms of amplication, the expansion is from a predetermined low value. as indicated by curve 62 in Fig. 9, to a higher value, as indicated at the time T5. Thereafter the gain rapidly rises, as shown by the broken line curve 61, reaching the maximum at time T6.

If the capacitor. 28 had been omitted throughout the aforesaid cycle of operations, the characteristic curve would then be as shown by the curve 68 of Fig. 8, with a corresponding amplification as shown by the curve 69, Fig. 9. Thus the effect of opening the contact 64 and removing the capacitor 28 from the circuit changes the operation from one characteristic curve to the other characteristic curve. The same result may be achieved by decreasing the resistance in circuit with capacitors 28 and 29, although this arrangement has. not been illustrated. F01` such a modification, the contact 64 of relay 63 would be arranged to change the effective position of the tap 34 to change the resistance, in circuit with capacitors 28 and 29, from 'an initial high value to a predeterminedy low value, the effect of which would be to transfer the operation from curve 34e, taken by way of example, to curve 34h, of Fig. 3.

' The operating coil of the relay 63 may be energized under the control of a gas tube, or Thyratron 18. In this case, a negative bias, derived from resistor 3|, is utilized normally to bias the control grid of the Thyratron 10 to prevent ow of anode current.A To control the time at which circuit constants are selected so that as the negaby interfaces at substantial depths, such as ve tion with the amplitude of the reflections received.

As soon as the need for contraction haspassed,

it is desirable immediately to provide maximum amplification for the following reflections of lesser amplitude.

For simplicity, provisions for the foregoing are tive voltage across resistor 3| decreases to a value at time T5, Fig. 8, the Thyratron B8 fires. Current then ilows from the battery 26 by way of the operating coil of relay v63, the Thyratron 10, and thence by conductors 32 and 32a to the negative side of battery 26. The relay 63 is preferably a high speed relay which operates immediately to open the contact 64 thus removing the capacitor 28 from the circuit and causing the above described rapid decrease in the gain controlling voltage Vs from its negative value.. This produces the rapid rise in the amplification, which has also been described.

With the above understanding of the simplified forms of the invention, it will be understood that many modificationsl and permutations may be 'made without departing from the spirit of the and in lieu of the utilization of a change of the bias on the suppressor grids, the amplification characteristics of the amplifier may be altered in other ways understood by those skilled in the art.

`The manner in whichthe several features ofi the invention thus far described are oombinedsil `into a `singleunitary system or master controllergi. will now be explained in connection withFig; 10.' The system of Fig. combines the several feai tures illustrated in Figs. l, 4,1 and 7. It will loev 28and 28. Theresistor 21 `is provided with three l tapswith Which cooperate a contacter 85. This contactor 85 is mechanically interlocked4 with contactors 861` and 81 respectively associated with capacitors 28 and 28, and .with contactor 88 `associated with resistor 9 l. The contactor 41 is shown in` full lineas in an o position, and by broken lines it is shown in its first position in which the input circuit for the gas tube 53 is completed;

In this manner, the contacter `41 performs the function of the switch 48 of Fig. 4. Y

`The contactors 85-88 areal] shown in their No. 1 positions. It Will be observed the capacitor 28 is not connected in circuit with the con- 4tact 64 of the relay B3. Instead, the contact G4 isconnected. to the deck 8| of the gang switch. In the `iirst position from off position of the gang switch 88--83 the decks 80 and 8l move to the right.. and. in doingso the capacitor 13` ls inserted in a series-circuit which includes con-` tact 64 of the relay 63'.

With all parts in the illustrated positions the Thyratron 53 will not re since contact 41' is in itsrst, the oi position. Hence, relay 42 cannot be closed. Therefore amplication is at a maximum since resistor 3| is excluded from the circuit and Vs is zero. The manual Vswitch 94 may be moved to itsvertical position to connect resistor 3| in the circuit in place of resistor 44i and independently of relay 42. The system willv then function to control the expansion underthe control of contactors 33 and `34iand their asso-` elated circuits, but the relay 42 will be ineilec tive.

There are several independent controls. adjustment of contactor 33 vdetermines themagnitude` of the initial negative bias developed between" the contactor 33 and ground and applied tothe suppressor grids. The adjustment of con:-` tactor 34 predeterminesthetime during which expansion of the amplification takes place.

Thev

lprovided by thevmeclianically-interlocked 3-posi Therefore, contactor 34 may be calibrated in seconds; that is, when it is moved from .6V to4.0,

maximum amplification will be that selected', as six-tenths of a second, four seconds, or as shown, 1.4 seconds.

Advantage istaken of the use of the negative bias from resistors 3l and 441to prevent the firing of` the Thyratron 18. Since the time-determining contactor 34 varies the time" of maximum amplication, it will be seen that the time of the firing of Thyratron 18 will vary therewith. If the time is long. the tiring of tube 10 will occur at a proportionately later time. Therefore, by providing the resistor 12 with a series of taps arranged for cooperation with the deck 83 of the gang switch 88-83, the positions of the` gangY switch may be calibrated in terms of the per cent ofthe time, selected by the positions ofthe contactor `34; The inclusionof apotentiometer' circuit in the gridclrcuitnofThyratron.18- enables the potential ,.ofthegrldof. tube-:lll` to'be varied, in@ ordern to' produce@.iiringz of 'the ,Thyratron 10 1 at the vdesiredpercentage =of..`.the ,time determined bythe position offtlcieacontactor 34; Forexarnple, with the gangtswitchf-88`83 initsrst position, the'Thyratron' res"afterexpirationiof (.4) forty percent of thetime -selected by contacter 34. This dependentl or-proportional relation makes possible the calibrationv of the gang switch` in terms of per cent of "the/ time-selected by the contacter 34;-

With the gang=switch18lil83in its final (.9) position, the Thyratron 10"'res after expiration of 90%` of .thetime selected by thecontactor 34. Thus, with theagang.switchxinN its firstv position, the relayy E3L.operates.in-.4 1.4=.56seconds after Thyratron 24,` while-insthelast position the relay operates on,.9, 11.4==1.26"seconds afterzthe firing of theTl'iyratron24;

Even with the relativelyl wide flexibility thus described, the. total time from the shot instant, or the ringiot Thyratron' 24, toY the. time of maximum amplification remainsv substantiallyv un changed. Th-isris accomplishedby changing the circuit" constants -andfapplied, Voltage by means of the decks 88, 8l, and 82." Brieiigv,` asthe gang switch is moved to increase-the per cent time (from .4to` .9) the decks and 8l connect the capacitol` 131inseries with the contactor- 64. In the next positionthe` capacitors 13 andv 14 are included in'that circuit, and inY the final Aposition all ofthe capacitors13-18 are inl circuit therewith. In consequence, morecapacityV is removed from theftime-delay'vcircuit by relay'83as the per cent time is increased; The removal, ordecrease. of the capacitance-decreases ,the timerequired'for Vs to reaclrgzero; Th'us. the relative values areselected so .that the overall time remains substantially constant.

Thereis also.an.independent adjustment for the contactar 41 along.resistor148,'which controls the initial time` during which the ampliiication is maintained: ata maximum for recordation vof the Vfirst breaks duetotheedirect traveling `.seismic waves.; The'- position of"A contactor4T-along resistor 48 may be"calibrated' infractions of a secondas fromfonez-tenthito. six-tenths of a second.` A

A further control* feature of the invention is tion .switcl:1es:-85`,83:Y When .thelcontactor 85v is moved` fromits. iirst ato., its zsecond i position (with deck 82,-insanyposition:from..4to -.9) a. smaller portion of the resistor 21 is, connectedacross the network. The reduction indthis'; resistance re, ducestheappliedvoltage and results in a greater reduction in .amplification duringthe period of time determinedzbyj contactors 8|,- 82',4 and 83; A further reduction in. yampliication occurs'upon movementofcontactor: fto its No. 3' position.

The increase inuVs also: appears. in the input, or`

control, circuit of the -'I'hyratron;18.` However, the contactorf88adjusts.the potential drop across the stepped resistor 12f in a.. direction such that the Thyratron is vfired, at the proper time corresponding to the position of contactors 33 and 34'.

The-contactors 881 and 81"servefii'st`to include the capacitory 28"an'cl-i finallyf the capacitor 29 in circuit with relay contact 64, morder to keep the overall expansionrv time .constantat the value indicatedl by contactor A341.

The foregoingrisillustrated` in the graphs of i lll Figs. 11 and 12. With the 3-position switch in its No. 1 position, it will be observed that the negative biasing voltage Vs has an initial value indicated at -V. This voltage rises in accordance with the curve 85a until the time indicated at T1. At'this instant the relay 63 operates to removefrom the circuit those capacitors which have been connected in series with the contact 64. Thereafter, the biasing voltage Vs rises at an increased rate, reaching substantially zero at the time T8. By reducing the potential difference, as by moving the 3-position switch 85--88 to its No. 2 position, the discharge of the capacitors occurs at a lower rate, as indicated by the curve 85h. However, in the second, or No, 2.

position the capacitor 28 is'included in circuit' may be selected, or predetermined, in accordance with the conditions imposed by the area over which the seismic survey is being made. Even though the rate of change over the initial period .may be adjusted, the maximum amplification is attained during the same time interval in both cases. This is clearly shown by the corresponding curves 85e and 85d, where maximum gain of amplication is attained at time T8. In the third position the rate of rise of the negative bias is at a much lower rate, as indicated by the curve 85e. However, in this position of the switch both o1' capacitors 28 and 29 are now included in circuitwlth the contact 64 of relay 63. Hence, when this relay operates at the time T1, both capacitors are removed from the circuit and the rate of rise of the negative bias in a positive direction is at a. very high rate, and adequate to insure that it reaches zero at the time T8, as before. Similarly, the corresponding gain curve 85! shows the gain gradually increasing to the time T1, with a very rapid expansion of ampliflcation Auntil it reaches maximum at thetime T0, asbefore.

It will now be seen that the curves shown in Figs. 2, 3, 5, 6, 11 and 12are illustrative of but a few of the large family of characteristic curves made possible by the relatively simple adjustments which have beendescribed. With many differing basic characteristic curves, the many additional adjustments provide for nearly every desired operation. The time for the initial contraction may take place anywhere from zero (the time of the shot instant or the time of firing of tube 24) to six-tenths of a second, and, with all of the variablepredetermined conditions of operation during the expansion, the overall expansion time may be the same or made constant regardless of wide changes in the other characteristics. i

Without repeating the previous detailed description, it may be observed-that movement of the gang switch 80--8'3 to the first or .4 position, through contactor 82, removes the lowermost section of resistor 21 from the network connected between contactor 86 and conductor 32a. At the same time, the first section of resistor 12 is inserted in the grid circuit of Thyratron 10. This compensates for the reduced applied voltage and the reduced rate of voltage change across resistors 3| or 44, and, as already explained, the capacitor 13 is placed under the control of the relay 63 in order that the total time shall be 'tactor 91, the first position of which is an off position, while the second and third positions correspond to the two positions shown for the switch '94 of Fig. 10. In Fig. 13 the resistors 3| and 44 are provided with contactors 98 and 99 mechanically connected together for simultaneous movement. Thus, by changing the position of contactors 98 and 99 with respect to resistors 3| and 44, the value of the gain controlling voltage Vs is increased or decreased, depending upon the direction of the adjustment. 'I'his gain control voltage is applied to a resistor |00. which serves as a multiple potentiometer arrangement for each amplifier. As is well understood in the art, each geophone is preferably provided with its own amplier, and the gain control therefor is preferably adjusted separately from the other amplifiers. For example. a number of amplifiers are symbolically designated at IDI, |02, |03, |04 and |05. By moving the contactor |06 downwardly to a position corresponding with the one marked 4, a predetermined negative bias will be applied to the suppressor grids of the amplifier |0|. AIf the geophone corresponding with the amplifier |02 receives early reflections whose amplitudes are somewhat different than those received by the geophone associated with the amplifier |0I, then the contactor |01 may be moved to a different position, such as position 3 or position 5. Similarly, the contactors |08, |09 and ||0 may be set for the desired magnitude of the initial suppressor voltage. In this way the amplifier for each geophone of the spread may be separately adjusted without affecting the operation of any of the other amplifiers or of the ampliers taken as a group.

It is to be understood that each of the taps ||3 are connected in the manner illustrated for the taps |-3. Further, it is to be understood that many more geophones and amplifiers than the ve, IUI-|05, are generally used. For simplicity only a few have been indicated, but as many as may be desired may be used by simply increasing the number of contactors corresponding with |09-I I0 and by extending the taps for cooperation therewith.

It will also be seen that the magnitude of the gain-controlling bias may be adjusted simultaneously for all the amplifiers1 by means of the contactors 98 and 99. With these various provisions adjustments can` be made to take care of widely differing terrain, which, in terms of a seismic survey, means that the data may be distinctively recorded and interpreted with greater accuracy. In many cases these adjustments make possible the securement of satisfactory data, which might otherwise be unattainable for a locality.

While the foregoing description is largely in terms of the control of the gain or amplification of the geophone amplifier, the invention as applied to the seismic recording system may be considered in terms of the sensitivity thereof. This recording system, including the geophone, the amplifier and the recording galvanometer eleforming a potentiometer.

ment which produces the seismograrn, is provided with an adjustable sensitivity which is varied in accordance with the intensity of the seismic signals to be recorded. In effect, the gain-controlling network, Fig. 10, between the conductors 32a and 32h comprises the section' of resistor 30 in series with the several capacitors, vwhich are connected in parallel with each other. 'I'he resistors 30 and 3| are connected directly across the network. They could, of course, be replaced by a single resistor. In other words, they may be consideerd as a single resistor insofar as they provide a constant resistance across the applied sources of potential difference. Considering them in this way, they may be considered as The lower part of this potentiometer, or resistor 30-31, is utilized to control the decay of the initial negative bias or the charge applied to the capacitors. A

It is to be further observed that the application of the second potential difference of opposite polarity forces, or hastens, the decay, or discharge, of the capacitors. Inasmuch as this second applied potential difference may be controlled by ccntactor `85, the permissible variation is over a range materially and substantially greater than if such forced discharge were not utilized. Moreover, as will be observed from the various curves, Figs. 2, 3, 5, 6, 8, 9, 11 and 12, the

contraction and expansion of the amplification, as well as the rate of rise of the biasing voltage Vs, is relatively uniform. While not linear, the straighter portions of the exponential rates of decay are, to a large extent, utilized, though the amplification may be expanded, as already explained, so that the lower rate of decay of the negative biasing Voltage Vs may be utilized.

While a preferred embodiment of the invention has been described, it will be understood that modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a seismograph system having a detector of seismic waves transmitted through the earth, an electrical recorder of said waves and a thermionic amplifier interconnecting said detector and said recorder, said amplifier having means responsive to a negative bias for controlling the contraction and expansion of the amplification thereof, the combination of aresistor, a source of potential for producing a potential'difference across said resistor, means for applying this potential diiference to said amplifier as said negative bias, capacitors, a charging circuit for said capacitors including said source of potential and a part of said resistor, means for applying to said resistor and to said capacitors a potential difference of polarity opposite to that of said source to produce a forced decay of said negative bias to zero and thence to a positive value, means for producing said decay at different selected time rates between a low limit of high negative bias and a high limit of zero bias without changing the time interval between said low limit and said high limit, and means for changing the initial high negative value of said bias without changing the time at which said bias is reduced to zero.

2. In a system having an amplifier and a recorder for recording signals applied to said amplifier, said amplifier having means responsive to a negative bias for controlling the contraction and expansion of the amplification thereof, the combination of means including a timing circuitl tors therein for applying to said amplifier a relatively high negative bias to produce substantial contraction of the amplification, means including said capacitors for decreasing said negative bias at a predetermined time rate to produce during a predetermined.v time interval an expansion of the amplification of said amplifier from a predetermined low limit to a predetermined high limit, means operable at any one of a plurality of selected times within said time interval for diierently increasing the time rate of expansion of said amplification in manner such that the length of said time interval is constant regardless of which of said times is selected, said last-named means including a switch for removing from said circuit an increasingly larger number of said capacitors as said selected time approaches the end of said time interval.

3. In a process of seismic prospecting in which the amplitude of seismic signals received at a recording system decreases as a function of time and in which an amplifier is provided to increase the amplitude of said signals, the combination of means for controlling the gain of said amplifiers at'a predetermined time rate comprising a resistor, means for applying a predetermined voltage across said resistor, means including a capacitor for controlling the rate of rise of voltage across said resistor, said capacitor being connected in series with at least a part of said resistor and in shunt with the remaining portion of said resistor, means for changing the amount of said resistor in circuit with said capacitor thereby to change the rate of rise of voltage across said resistor Without changing the initial value thereof, and adjustable means for deriving from said resistor a gain-controlling voltage, the adjustment of said means with respect to said resistor predetermining such initial gain-controlling voltage.

4. In a system of seismic prospecting in which the amplitude of seismic signals received at a recording system decreases as a function of time and in which an amplifier is provide to increase the amplitude of the received signals, the combination of means for controlling the gain of said ampliiier at a predetermined time rate comprising a resistor, means for producing a predetermined voltage across said resistor, a second resistor identical with said first resistor, means for producing a voltage rise across said first resistor from a predetermined negative value to a lower negative Value and thence to a positive Value, means operable at a predetermined time for substituting said second resistor for said first resistor, means including said second resistor for applying a gain-controlling voltage to said amplifier to suppress the gain thereof and thereafter to increase the gain thereof.

5. In a system of seismic prospecting in which the amplitude of seismic signals received at a recording system decreases as a function of time and in which an amplifier is provided to increase the amplitudeof the received signals, the combination of means for controlling the gain of said t amplifier at a predetermined time rate comprising a resistor, capacitive means included in circuit with said resistor, means for deriving a gaincontrolling biasing potential from a part of said resistor, means for initially establishing a predetermined gain-suppressing biasing potential across said resistor, means for changing said biasing potential at a predetermined time rate and in a direction to' increase the gain of said amplifier, and means operative at a `predeter minedtime interval, after said biasing potential has risen a predetermined amount, for substantially decreasing the capacity of said capacitive means, thereby abruptly to change said biasing potential in a direction rapidly to increase the amplification of said amplifier.

6. In a seismograph system having a detector of seismic waves transmitted through the earth, an electrical recorder of said Waves, and a thermionic amplifier interconnecting said detector and said recorder, said amplifier .having means responsive to a negative bias for controlling the contraction and expansion of the amplification thereof, thev combination of a network comprising a circuit including a plurality of ycapacitors in parallel with each other and in series with a resistor, a second resistor in series with said first resistor and connected across said network independently ofthe circuit including said capacitors, means for applying to said amplier a bias from said second resistor, a source of supply for apply.. ing a potential of one polarity to said network negatively to charge said capacitors and to produce across said second resistor a negative bias of magnitude adequate greatly to reduce the amplification of said ampliiier, means for applying to said network a potential of opposite polarity for reducing said negative charge of said capacitors, thereby to decrease said negative bias at a predetermined time rate, means operable a time interval after application of said second potential to `said network for reducing the number of capacitorsconnected across said network to change the time rate of decrease of said negative bias, means `for varying said time interval, and switching means operable concurrently with said lastnamed means for increasing the number of capacitorsexcluded from said network as said time interval is increased.

'1. In a system having an amplifier and a recorder for recording signals applied to said am- 'pliiier, said amplifier having means responsive to a negative bias for controlling the contraction and=expansion of the amplification thereof, the combination of means including a timing circuit having capacitive means therein for applying to said amplifier a Vrelatively high negative bias to produce substantial contraction of the ampliiication,`means including said capacitive means for decreasing said negative bias at a predetermined time rate to produce during a predetermined time interval an expansion of the amplification of said amplifier yfrom a predetermined low limit to a predetermined high limit, means operable at any one of a plurality of selected times within `said time interval for differently increasing the time rate of expansion of said amplification in manner such that the length of said time interval is constant regardless of which of said times is selected, said `last-named means including means for removing from the circuit an increasingly greatenproportion of the capacitance of said capacitive means as said selected time approaches the end of said time` interval.

l 8. lna'system having anampliiier and a recorder for recording signals applied to said amplifier, said amplifier having means responsive to a negative bias for controlling the contraction and `expansion of the amplification thereof, the combination of means including a timing circuit having a plurality of parallel-connected capacitors therein for applying to said amplifier a relatively high negative .bias to produce substantial contraction ofthe ampliiication, means including said capacitors for decreasing said negative bias at a predetermined time rate to produce during a predetermined time interval an expansion of the amplification of said ampliiier from a predetermined low limit to a predetermined high limit, means for varying within relatively wide limits said time rate of expansion of said amplification, means operable at a predetermined time interval after expansion of said amplification at a selected rate for increasing the time rate of further expansion of said amplification to a degree such that the length of the time interval between said low limit and said high limit is constant regardless of the rate selected for said initial expansion of said amplification, said last-named means including means for removing from said circuit an increasingly larger number of said capacitors as said selected time rate of initial expansion is made less.

9. In a system having an ampliiier and a recorder for recording signals applied to said amplifier, said ampliiier having means responsive to a negative bias for controlling the contraction and expansion of the ampliiication thereof, the combination of means including a timing circuit having a plurality of parallel-connected capacitors therein for applying to said amplifier a relatively high negative bias to produce substantial contraction of said amplification, means including a selector switch for varying the number of said capacitors in said timing circuit thereby to change 'the time rate of decay of said negative bias to ing a switch for removing from said timing circuit a predetermined number of said capacitors, an electric valve for controlling operation of said switch, means for applying said negative bias to said valve initially to render it non-conductive and means operable concurrently with said selector switch for so changing the bias on said valve as to render said valve conductive at the end of a predetermined time interval regardless of the selected rate of decay and of the magnitude of said negative bias.

10. In a seismograph system having a detector of seismic waves transmitted through the earth, an electrical recorder of said waves and a thermionic amplifier interconnecting said detector and said recorder, said amplifier having means responsive to a negative bias for controlling the contraction and expansion of the ampliiication thereof, the combination of a resistor, a source of potential for producing a potential difference across said resistor, means for applying this potential difference to said amplifier as said negative bias, capacitive means, a charging circuit for said capacitive means including said source of potentialand a part of said resistor, means for applying to said resistor and to said capacitive means a potential difference of polarity opposite to that of said -source to produce a forced decay of said negative bias t0 zero and thence to a positive value, means for producing said decay at different selected time rates between a low limit of high'negative bias and .a high limit of zero bias without changing the time interval between saidr low limit and said high limit, and means for changing the initial high negative value of said bias without changing the time at whic Number said bias is reduced to zero. Y 2,317,334 2,321,341 EDWIN J. SHIMEK. 2,276,709 GOETHE M. GROENENDYICE. 5 2,276,708 2,312,934 REFERENCES CITED 2,286,170 Thefollowing references are of record in the 23541420 me af this patent: lo 2.3761195 UNITED STATES PATE'N'IS Number Name Date Number 2,301,739 Minton, et al. Nov. 1o, 1942 536,747

Name Date Shimek Apr. 20, 1943 Weatherby, et a1. June 8, 1943 Wyckoff Mar. 17, 1942 Wyckoff Mar. 17, 1942 Shook Mar. 2, 1943 Heiland June 9, 1942 Minton July 25, 1944 Scherbatskoy May 15, 1945 FOREIGN PATENTS Country Date Great Britain May 26, 1941 

